Astronomy: An all-American eclipse : Nature : Nature Research – Nature.com

David Baron Liveright: 2017. ISBN: 9781631490163

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The progress of an eclipse seen from Australia in 2012.

On 21 August 2017, the United States will experience its first all-American total solar eclipse. The path of totality's full shadow some 100 kilometres wide will for the first time make landfall only in the United States, passing over the homes of 12 million people in 14 states, from Oregon to the Carolinas. Heliophysicists and umbraphiles from around the world are preparing for it, along with Department of Transportation officials. The former are still pondering the results of the 1999 eclipse, whose path crossed Europe from Cornwall to Romania and beyond, and of total eclipses since. The latter are doing their best to ensure that millions of drivers get safely into and out of the path.

A total solar eclipse is the most stupendous sight in nature.

The fuss is understandable. A total solar eclipse is the most stupendous sight in nature: the abruptly darkening sky; Baily's beads, glints of sunlight shining through lunar valleys; the dazzling diamond-ring effect; the spiky, pearly solar corona. Then, a couple of minutes later, the whole show in reverse. Equally compelling is the knowledge that you are witnessing a syzygy, an alignment of Earth, Moon and Sun that darkens the sky by an additional factor of 10,000 in the last minute alone. Now, four books all anticipate the coming celestial event in different ways.

In American Eclipse, journalist David Baron harks back to the total eclipse visible in the United States in July 1878. (I read this book in draft and provided a blurb.) A group of eminent scientists, including astronomer Henry Draper and his wife, Anna (see S. Nelson Nature 539, 491492; 2016), travelled to Rawlins, Wyoming, to witness it. But, as Baron relates, 31-year-old whizz-kid inventor Thomas Edison gained the lion's share of publicity, even though he was just tagging along. Edison brought one of his devices, a tasimeter, to measure minute shifts in heat from the Sun's corona during the eclipse. He was unprepared for the strength of the signal, however, and his instrument's needle pinned at its maximum reading. It wasn't until around 1940 that physicists Walter Grotrian, Bengt Edln and Hannes Alfvn found the solar corona to have a temperature of at least 1 million C. Had the tasimeter worked, the scattering of sunlight that we see as the inner corona would have misleadingly given Edison the Sun's surface temperature, 6,000 C.

Baron's stories are good ones, well told. The pioneering US astronomer Maria Mitchell the first professor hired at Vassar College in Poughkeepsie, New York took a group of alumnae, although they weren't offered free rail travel like their male counterparts. Astronomer and inventor Samuel Pierpont Langley, meteorologist Cleveland Abbe and solar spectroscopist-astronomer Charles Young also witnessed the eclipse. Nine years before, Young had co-discovered the green line in the spectrum of the corona that proved key in understanding coronal temperature; in the 1940s, it was found to come from iron gas so hot that many of its atoms have lost half their electrons.

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Astronomy: An all-American eclipse : Nature : Nature Research - Nature.com